The current mobile communication network has a format in which a synchronous mobile communication system (CDMA mobile communication system) called 2G or 2.5 G communication networks and an asynchronous mobile communication system (WCDMA mobile communication system) called 3 G communication network co-exist.
As such mobile communication network technology has advanced, a dual band and dual mode terminal (DBDM), which can be used in both the synchronous system and the asynchronous system, is being currently developed, and such mobile terminal allows the use of different modes of services in both the asynchronous system area and the synchronous system area. The DBDM mobile terminal comprises an antenna, a synchronous modem that is a module for a synchronous mobile communication service, an asynchronous modem that is a module for an asynchronous mobile communication service, and a common module.
Since the asynchronous mobile communication system is at an initial stage of services and needs massive investment costs to implement the system, it cannot provide coverage over a wide area, so it is implemented in the form of overlap with the synchronous mobile communication system area. Since the service coverage area of the asynchronous mobile communication system is limited, if a subscriber of the asynchronous mobile communication system, who is using the service in the asynchronous service area, moves to the synchronous service area where the asynchronous mobile communication service is not provided, a handover is needed to provide a continuous service to the subscriber.
Currently, a variety of handover methods are being studied for a terminal which moves from the asynchronous mobile communication system to the synchronous mobile communication system. During such a handover of a mobile terminal, the mobile terminal must switch an asynchronous vocoder, which is used for a voice call with the asynchronous communication system, to a synchronous vocoder, which is used for a voice call with the synchronous communication system, and both the asynchronous and synchronous communication systems require a vocoder switching process. Accordingly, both the mobile terminal and the mobile communication system can conduct a voice call after the vocoders are switched.
However, since there exists a gap between the timing of switching the vocoder of the mobile terminal and the timing of switching the vocoder of the mobile communication system, a silent period occurs during handover, and thereby the service quality is degraded.